1. Field of the Invention
The present invention relates to a magneto-optical recording medium for recording and reproducing information and which can be rewritten, more specifically a magneto-optical recording medium in which direct overwriting can be done by modifying the power level and/or pulse width of applying laser pulses, without changing the direction and intensity of a bias magnetic field.
2. Description of Related Art
Magneto-optical recording media have been widely investigated and developed as high density and high capacity information recording media. Particularly, rewritable magneto-optical recording media can be used in various fields and, therefore, various materials and systems have been proposed therefor and thus rewritable magneto-optical recording media have already made into use.
When compared with recording media such as floppy discs and hard discs, magneto-optical recording media have a drawback in that overwriting is difficult. Here, overwriting means that old information is simultaneously erased while new information is written.
Several overwriting technologies for magneto-optical recording media have been proposed, and one which has attracted attention is a direct overwriting system using a self-reversible magneto-optical recording medium in which the direction of net remnant magnetization in a portion of the magnetic wall boundary region heated by a laser beam can be reversed by modifying the power level and/or the pulse width of laser pulses between erasing and writing, without changing the direction and intensity of bias magnetic field, since it does not necessitate a significant modification of the optical system, magnet, etc. of a presently commercially available magneto-optical recording apparatus. For examples, see Japanese Unexamined Patent Publication (Kokai) No. 01-251357; J. Appl. Phys. Vol. 63, No. 8 (1988) 3844; IEEE Trans. Magn. Vol. 23, No. 1 (1987) 171; Appl. Phys. Lett. Vo. 49, No. 8 (1986) 473; IEEE Trans. Magn. Vol. 25, No. 5 (1989) 3530; and J. Appl. Phys. Vol. 69, No. 8 (1991) 4967.
A recording layer composed of a layer having a perpendicular easy magnetization axis and a layer having an in-plane easy magnetization axis is known, but the purpose thereof is to improve the sensitivity of the recording layer to the recording magnetic field, as described in Japanese Unexamined Patent Publication (Kokai) Nos. 02-247846 and 04-370550 and in Material for Study in Japan Society of Applied Magnetics.
When an in-plane magnetization layer is used in a domain wall-motion type direct overwriting recording medium, the purpose thereof is to prevent formation of the Bloch wall (Bloch line) by using a permalloy layer, an ion-implanted layer, a surface oxidized layer or a surface crystallized layer as an in-plane magnetization layer with an optimum thickness of 5 to 20 nm, as described in Japanese Unexamined Patent Publication (Kokai) No 63-241739. This publication mentions that the optimum thickness of the in-plane magnetization layer is 5 to 20 nm and an in-plane magnetization layer having a thickness of less than 2 nm does not have the necessary effect in preventing the Bloch wall.
A recording layer composed of a layer having a perpendicular easy magnetization axis and a layer having an in-plane easy magnetization axis is also proposed in Japanese Unexamined Patent Publication Nos. 05-120754 and 05-182264. However, the purpose of providing the layer having an in-plane easy magnetization axis is different from the present invention. In Japanese Unexamined Patent Publication No. 05-120754, a layer having an in-plane easy magnetization axis is magnetized in a direction parallel to the layer both at room temperature and at a raised temperature close to the Curie temperature. In Japanese Unexamined Patent Publication No. 05-182264, the layer having an in-plane easy magnetization axis is magnetized in a direction parallel to the layer at room temperature and in a direction perpendicular to the layer at a raised temperature close to the Curie temperature. Either magneto-optical recording medium provides as high a C/N ratio as that obtained in the present invention.
The present inventors have actually done experiments to evaluate the conventional direct overwriting method. The recording medium used comprised a polycarbonate resin substrate having a diameter of 130 mm and a thickness of 1.2 mm and having groups in the form of spiral with a pitch of 1.6 .mu.m thereon, and a transparent dielectric layer of AlSiN having a thickness of 120 nm formed on the substrate, which AlSiN layer was inverse-sputtered for 10 nm. A 25 nm thick rare-earth transition-metal amorphous alloy magnetic layer of (Gd.sub.25 Tb.sub.75).sub.24 (Fe.sub.90 Co.sub.10).sub.76, was then deposited as a self-reversible magneto-optical recording layer on the transparent dielectric layer. A 40 nm thick transparent dielectric layer of AlSiN and 60 nm thick a reflecting layer of AlAuTi were then deposited on the recording layer.
The direct overwriting characteristics of this medium were tested. The medium was set on a drive for evaluation. The rotation speed of the disc was 11.3 m/sec as a linear speed at a point 30 mm from the center of the disc. The recording and erasing were conducted using 7.4 MHz pulses as shown in FIG. 1 while applying an external bias magnetic field of 350 Oe in the direction of writing. The power of a laser beam, at a wavelength of 830 nm, was 8.0 mW for recording and 4.0 mW for erasing. Next, a 1.0 mW DC beam, a continuous wave beam, was applied to the disc to obtain a reproduction signal for evaluation. The reproduced signal had a C/N (carrier to noise) ratio of about 34.0 dB.
Recording and erasing were then conducted on the same track as used for the above evaluation, using 5.5 MHz pulses as shown in FIG. 2 while an external bias magnetic field with an intensity of 350 Oe was applied thereto in the direction of recording. Namely, the signal as shown in FIG. 1 was overwritten by the signal as shown in FIG. 2. The measurement of reproduced signal was then conducted by applying a DC beam, i.e., a continuous beam with a power of 1.0 mW. As a result, it was confirmed that the 7.4 MHz signal originally recorded was erased and only the 5.5 MHz signal was recorded. The C/N ratio at this time was about 34.0 dB.
As seen above, a direct overwriting operation by the above mentioned optical modification method was confirmed. However, the C/N ratios of recording and reproducing were as low as about 34 dB, indicating that a remarkable improvement in the C/N is necessary.
Next, a magneto-optical recording medium as mentioned above except that a 5 nm thick permalloy layer was added to the GdTbFe layer, was prepared and the overwriting characteristics of the disc were examined. The recording sensitivity as well as the C/N ratio of the medium were significantly reduced and an improvement in the C/N ratio could not be obtained.
The object of the present invention is to provide a magneto-optical recording medium in which direct overwriting can be done by modification of power level and/or pulse width of the applied laser pulses without changing direction or intensity of bias magnetic field, and in which the C/N ratio is improved.